The present invention relates to a small capacitance change detection device and, more particularly, to a small capacitance change detection device for detecting a surface shape having a small three-dimensional pattern of, e.g., the skin surface of a human finger or nose of an animal as a small change in capacitance.
As sensors for recognizing a surface shape having a small three-dimensional pattern, especially, devices aiming at fingerprint detection have been reported. As a technique of detecting a fingerprint pattern, a capacitive detection type sensor using the LSI manufacturing technology has been proposed. This is described in, e.g., xe2x80x9cISSCC DIGEST OF TECHNICAL PAPERSxe2x80x9d, FEBRUARY 1998 pp. 284-285.
A capacitive detection type sensor senses the three-dimensional pattern of the skin surface of a finger by detecting an electrostatic capacitance generated between the electrodes of small sense units two-dimensionally arrayed on an LSI chip and the skin of a finger in contact with the electrodes via an insulating film. Since the capacitance value changes depending on the three-dimensional pattern on the skin surface of a finger, the three-dimensional pattern of the skin surface of a finger can be sensed by detecting the small capacitance difference.
FIG. 54 shows the basic arrangement of a conventional small capacitance change detection device using this principle. This small capacitance change detection device has a detection element 310 formed from an electrostatic capacitance between an electrode and skin of a finger in contact with the electrode via an insulating film, signal generation circuit 320 for generating a voltage signal corresponding to the electrostatic capacitance value of the detection element 310, and output circuit 340 for converting the voltage signal from the signal generation circuit 320 and outputting a signal.
FIGS. 55A and 55B show the layout of the conventional small capacitance change detection device. This small capacitance change detection device has a plurality of detection elements 310, a plurality of signal generation circuits 320, and a plurality of output circuits 340. One detection element 310 and one signal generation circuit 320 construct a sense unit 301. The sense units 301 are two-dimensionally arrayed on an LSI chip to form a sensor array 302. The output circuits 340 are arranged near the sensor array 302 to form an output section 304.
Since the electrostatic capacitance value of each detection element 310 is determined depending on the distance between the electrode of the sense unit 301 and skin surface of a finger, the electrostatic capacitance value of the detection element 310 changes depending on the three-dimensional pattern of the skin surface of the finger. When a finger is depressed against the sensor array 302, each sense unit 301 outputs a voltage signal corresponding to the three-dimensional pattern of the skin surface of the finger. This voltage signal is converted into a desired signal reflecting the three-dimensional pattern of the skin surface of the finger, so the fingerprint pattern is detected.
The arrangement and operation of the conventional small capacitance change detection device shown in FIG. 54 will be described below in more detail.
FIG. 56 shows the circuit arrangement of the conventional small capacitance change detection device. Referring to FIG. 56, reference symbol Cf denotes an electrostatic capacitance formed between the electrode of the sense unit 301 and the skin surface of a finger in contact with the electrode via an insulating film. The electrode of the sense unit 301 is connected to the input side of a current source 321 of a current I through an NMOS transistor Q3. A node N1 between the electrode and transistor Q3 is connected to the input side of the output circuit 340. A power supply voltage VDD is applied to the node N1 through a PMOS transistor Q1. The node N1 has a parasitic capacitance Cp1. Signals {overscore (PRE)} and RE are supplied to the gate terminals of the transistors Q1 and Q3, respectively.
The capacitance Cf forms the detection element 310. The current source 321 and transistor Q3 construct the signal generation circuit 320.
FIGS. 57A to 57C explain the operation of the small capacitance change detection device shown in FIG. 56.
First, the signal {overscore (PRE)} of high level (VDD) is supplied to the gate terminal of the transistor Q1, and the signal RE of low level (GND) is supplied to the gate terminal of the transistor Q3. Hence, both the transistors Q1 and Q3 are OFF.
In this state, when the signal {overscore (PRE)} changes from high level to low level, the transistor Q1 is turned on. Since the transistor Q3 is kept off, the node N1 is precharged to VDD.
After precharge, the signal {overscore (PRE)} goes high, and simultaneously, the signal RE goes high. The transistor Q1 is turned off, and the transistor Q3 is turned on. Charges stored at the node N1 are removed from the current source 321. As a result, the potential at the node N1 lowers.
Letting xcex94t be the period while the signal RE is at high level, a potential drop xcex94V at the node N1 after the period xcex94t elapses is given by Ixcex94t/(Cf+Cp1).
Since the current I, period xcex1t, and parasitic capacitance Cp1 are constant, the potential drop xcex94V is determined by the capacitance Cf. Since the capacitance Cf is determined by the distance between the electrode of the sensor and the skin surface of a finger, the value of the capacitance Cf changes depending on the three-dimensional pattern of the skin surface of a finger. This means that the magnitude of the potential drop xcex94V changes reflecting the three-dimensional pattern of the skin surface of a finger. This potential drop xcex94V is supplied to the output circuit 340 as an input signal. The output circuit 340 identifies the magnitude of the potential drop xcex94V and outputs a signal reflecting the three-dimensional pattern of the skin surface of a finger.
In the conventional small capacitance change detection device, however, when the parasitic capacitance Cp1 at the node N1 is large, the potential drop xcex94V becomes small. When the circuit shown in FIG. 56 is arranged using the LSI manufacturing technology in practice, the parasitic capacitance Cp1 becomes larger than the capacitance Cf.
The potential drop xcex94V can be made large by increasing the current I of the current source 321 or period xcex94t of the signal RE at high level. However, when the current I is large, the sense units 301 with manufacturing variations are hard to control. For this reason, the current I is preferably relatively small to obtain high detection accuracy. Also, the period xcex94t cannot be made so long from the viewpoint of the detection time.
Consequently, the potential drop xcex94V as a signal to be input to the output circuit 340 becomes small, and the output varies due to noise margin or manufacturing variations, resulting in a decrease in surface shape detection accuracy.
Hence, as described above, a signal change reflecting the three-dimensional pattern of a skin surface of a finger decreases due to the influence of a parasitic element such as the parasitic capacitance Cp1 formed in the manufacturing process, and the detection accuracy of the small capacitance change detection device becomes low.
It is therefore an object of the present invention to accurately extract a small change in capacitance by a small capacitance change detection device.
It is another object of the present invention to increase the design margin of the output circuit of a small capacitance change detection device.
In order to achieve the above objects, according to the present invention, there is provided a small capacitance change detection device comprising a capacitance detection element for detecting a small capacitance change, a signal generation circuit having an output side connected to the capacitance detection element to control predetermined charges, a signal amplification circuit having an input side connected to a connection point between the output side of the signal generation circuit and the capacitance detection element, and an output circuit connected to an output side of the signal amplification circuit, the signal amplification circuit comprising a first transistor having one output terminal connected to the connection portion between the output side of the signal generation circuit and the capacitance detection element, a first voltage source connected to a control terminal of the first transistor, a second voltage source, and a third voltage source, wherein one of the second and third voltage sources is connected to the other output terminal of the first transistor via a first switch, a voltage to be applied from the second voltage source to the other output terminal of the first transistor is set to have a value not less than a value obtained by subtracting a threshold voltage of the first transistor from a voltage of the first voltage source that is applied to the control terminal of the first transistor while a voltage to be applied from the third voltage source to the other output terminal of the first transistor is set to have a value not more than a value obtained by subtracting the threshold voltage of the first transistor from the voltage of the first voltage source that is applied to the control terminal of the first transistor, and the output circuit is connected to a connection point between the other output terminal of the first transistor and the first switch and, after a voltage of one of the second and third voltage sources is applied to the connection point in an ON state of the first switch, receives the voltage at the connection point on the basis of an OFF state of the first switch and charge control by the signal generation circuit after the first switch is turned off.